1. Field of the Invention
This invention relates to implantable drug delivery techniques, and more particularly to such techniques in which fluid from a body is used to provide a force which delivers a treatment agent.
2. Description of Related Art
Drug delivery systems based on osmotic pumps are known and are described, for example, in U.S. Pat. No. 4,624,847 (Ayers et al.), issued Nov. 25, 1986. When using such an osmotic pump, it is impossible to control the rate of hydration of the osmotic medium. In known osmotic pumps, a porous membrane separates a hygroscopic media from body fluids. As the membrane passes water from the body into the pump due to osmosis, the media swells and forces the infusate out of a reservoir. Such an osmotic pump is a passive device that has a fixed infusion rate and no on/off control. This invention addresses the fixed infusion rate problem of known osmotic pumps which limits their therapeutic value.